Phoseon Technology: UV LED Curing for Pinning - Controlling Image Quality through Pinning

UV LEDs light sources allow digital printers to gel or freeze UV ink droplets quickly before they have time to spread out (referred to as drop or dot gain). The process of freezing ink droplets between print heads is called pinning. This is possible because the UV LED light sources are more compact than conventional curing systems and can be easily positioned between successive print heads. While pinning stops the drop from spreading, it leaves the ink flexible and soft enough for proper inter-coat adhesion to other ink droplets and for further handling. All pinned inks must finally pass underneath a final cure UV LED light, to finish the curing process.

Full or final curing ordinarily occurs at the end of the process and transforms the ink into a more rigid film with the required final surface properties. All pinned inks must still pass underneath a final cure UV LED light source to be fully cured.

Pinning Process

Pinning is also different from so-called inter-deck curing found on some larger offset printing presses, where colors are nearly completely cured near the drum before other colors are applied downstream. A close-to-full cure is necessary with an offset process because the transfer plates make physical contact with the cured surface during printing. Pinning is possible with inkjet processes because inkjet printing requires no physical contact with the cure surface. The extent of dot or dot gain depends on properties of the ink, substrate, and the process environment. The advantages of pinning with UV LEDs include:

• Control of Dot Gain

• Improved image quality & line resolution

• Reduce Banding

• Leaves ink flexible

• Better Adhesion of additional colors - for both absorbent and non-absorbent substrates

• Small form factor lamps

• Low heat impact

• Instant On/Off

• Intensity (pinning) control

• Low voltage

• Long UV LED lamp life (+20K hours of On-time <10% degradation)

• Stable & predictable output

Dot Gain

Just as it’s easier to shoot water from a squirt gun than it is cream cheese; thinner, less viscous inks jet more easily. UV jettable inks are typically in the 10-15 cP viscosity range and tend to spread more easily than thicker inks. Any number of formulating tweaks for color, gloss, or rheological properties can have unintended consequences when it comes to dot gain, and so there’s not likely to be a one-size-fits-all setting. Dot Gain can cause:

• Blurred images

• Muddled colors

• Loss of definition

• Lack of color integrity

• Poor gloss control

The substrate plays an equally important though often less predictable role in determining dot gain. While some materials absorb ink and have varying degrees of porosity, other materials present a slick surface making it easier for inks to spread around more easily. For example, coated papers designed to promote wetting also promote dot gain. With an ever-expanding choice of paper and plastic films and the advent of variable data printing systems designed for small production runs on a range of parts, it is unrealistic to expect a “set it and forget it” solution.

Other nuts and bolts of the process also affect dot gain—such as the web or press speed, the distance from the print head to the substrate, and the local airflow, heat and humidity. In the end, a daunting combination of variables from ink to substrate to process requires a pinning system that can be quickly, but precisely fine-tuned, sometimes color-by-color, to achieve the needed resolution. UV LEDs provide that level of simple, but effective control over the pinning process.

UV LED

Dot gain is a finicky problem, confounded by many variables, but one that is being helped by a new technology: Ultraviolet Light Emitting Diodes (UV LEDs). UV LEDs are light sources that allow digital printers to gel or freeze UV ink droplets quickly before they have time to spread out.

This is possible because the UV LED light sources are more compact than conventional curing systems and can be easily positioned between successive print heads. The process of freezing ink droplets between print heads is called pinning and is somewhat different from fully curing the ink (Figure 3). While pinning stops the drop from spreading, it leaves the ink flexible and soft enough for proper inter-coat adhesion to other ink droplets and for further handling. Full or final curing ordinarily occurs at the end of the process and transforms the ink into a more rigid film with the required final surface properties. All pinned inks must still pass underneath a final cure UV LED light source to be fully cured.

Difference From UV

As the name implies, UV LEDs are an ultraviolet variety of light-emitting diodes. They differ dramatically from traditional UV lamps used for printing both in how they produce UV, and in their benefits. Traditionally, lamp suppliers have produced UV by energizing a small amount of mercury that is sealed in a quartz tube. When high voltage is applied to the lamp’s electrodes, (or with microwave lamps, when the tube is placed in a microwave generator), UV (along with a large amount of visible light and infrared heat) is created. So, the output spectrum of mercury lamps contains many peaks over a broad range of wavelengths that extend from 200nm to well into the IR region above 1000nm.UV LEDs, on the other hand, are semiconductor devices; modern cousins of the LED on your garage door opener except that UV LEDs emit light in the longer-wavelength portion of the UV spectrum.

The exact output of the LED is chemistry-dependent, but typically the output is a relatively monochromatic or narrow peak of energy somewhere between 365 to 405nm

At first, the more specific, narrow bandwidth output of the LED caused UV chemists some fits as they scurried to adapt ink formulations to these new sources. But the benefits of UV LEDs for digital printing clearly made this effort worthwhile because UV LEDs offer many advantages that arc lamps do not. For example, they do not generate dangerous short wavelength UV that can be a safety hazard, and they do not produce ozone. They do not contain mercury, another potential safety hazard, or use high voltages.

LEDs are also much cooler than arc lamps, and they can be turned on and off instantly without the need for shutters. LEDs are compact in size and produce more uniform light sources that last much longer than arc lamps. But it is the ability to precisely control the UV output of the light source, along with their compact size, and cool-running temperatures that limit heat build, that make UV LEDs ideal for pinning inks. Compact UV LEDs can be fitted in tight confines between inkjet heads, a technique that would be impossible for arc lamps.

The use of state-of-the-art UV LED technology to improve dot gain is advancing digital printing on two related fronts. While pinning helps to produce higher quality images with existing technology, it also makes it possible for press designers to achieve a higher perceived resolution with less expensive or complex jetting hardware. After all, resolution is not determined by the number of dots applied per inch alone, but also by the distinctness of those droplets to the reader’s eye. If ink droplets are fuzzy, or mingled with others, even a high density of droplets per inch can be a wasted investment.

In Conclusion

While there is broad agreement that pinning improves image quality, there are still questions regarding the ideal settings needed for optimal pinning. Ink formulators, UV-pinning-system manufacturers, and integrators are experimenting with UV LED settings, timing, and ink sets to find the answers. The activity is a strong indicator that more and more systems are, and will be, incorporating pinning into the inkjet printing process.